1. Field of the Invention
The present invention relates to a substrate potential detecting circuit.
2. Description of the Prior Art
In general, a substrate potential detecting circuit is used as when a substrate potential generating circuit is controlled to be turned on or off according to the substrate potential. As shown in FIG. 9, a prior art substrate potential detecting circuit is a series circuit constructed by connecting n-units of resistances R.sub.1, R.sub.2, . . . , R.sub.n in series. One end of this series circuit is connected to a drive supply voltage V1; the other end thereof is connected to a substrate voltage V2; and an output voltage V.sub.out is derived from an intermediate junction node of the series circuit.
In the prior art substrate potential detecting circuit as described above, it is desirable to use high resistances to reduce the power consumption thereof. For instance, when the substrate potential detecting circuit is connected between a supply potential of 5V and a p-type substrate potential of 0 V and further when current consumed through the resistances is required to be reduced down to 0.01 .mu.A, the necessary resistance value is as high as 500M.OMEGA.. For this purpose, in general, a well having a high sheet resistance has been so far used. In this case, however, there exists a problem in that a large pattern area is required. For instance, when the sheet resistance of the well is 1 k.OMEGA., a layout pattern having a width of 1 .mu.m and a length of 40 mm is necessary, so that a pattern area of about 1 mm square is needed. On the other hand, since there exists such a tendency that the sheet resistance value of the well decreases with increasing microminiaturization of the device, this problem becomes serious more and more.
Further, when a signal varying very slowly (e.g., the substrate potential) is inputted to the substrate potential detecting circuit as an input signal, since the output is responsive to a slight potential variation (e.g., caused by noise) of the substrate potential, a resistance against noise is also required.